26 GEOLOGICAL SURVEY OF TIIE TERRITORIES. 



the mouth of the Yellowstone, on O'Fallon's Creek, one hundred miles 

 above the month of the Yellowstone, and in the valley of that stream. 



" The explorations of Dr. Hay den prove that this Miocene Lignite for- 

 mation occupies the beds of extensive lakes, which formed basins on the 

 surface of the continent when it had but recently emerged from the 

 Cretaceous sea. As has been remarked elsewhere, the lower members 

 of the series contain a few estuary shells, showing the access of salt- 

 water at that period ; but during tbe deposition of by far the greater 

 portion of these beds, the water of the ocean was entirely excluded from 

 the basins in which they accumulated. There is, therefore, every reason 

 to believe that the debris of ligneous plants which compose this collec- 

 tion were derived from trees which grew along the shores of the lakes 

 and streams of the Tertiary continent; that then, as now, alternations 

 of seasons prevailed, by which the foliage of these trees were detached 

 by an autumnal frost, and that falling into the water beneath or near 

 them, and sinking to the bottom, they were enveloped in mud, precisely 

 as leaves of our sycamores, willows, oaks, etc., accumulate at the bottoms 

 of our streams and lakes of the present day." 



I need not extend these remarks farther to illustrate the views of 

 both' paleontologists in regard to the age of the Lignitic group, as 

 observed in the Northwest, up to within a comparatively recent period. 

 1 need not refer to the views of Mr. Lesquereux, inasmuch as they have 

 been consistent in the belief of their Tertiary age, from the commence- 

 ment of his examination up to the present time, and his arguments in 

 favor of this belief have been set forth in nearly all the annual reports 

 of the Survey. 



If the Lignitic group, as developed on the Upper Missouri, is admit- 

 ted to be either entirely or in part of Tertiary age, the question will arise, 

 what bearing has this admission on the age of the coal-beds of Wyo- 

 ming and Colorado? 



I beg just here to call the attention of geologists to the geological 

 maps prepared by me, and published in the Final Eeport of Nebraska, 

 1869, and in the Geological Report of the Exploration of the Yellowstone 

 and Missouri Rivers, 1859-'60, especially the latter map. It will be seen 

 by the last-named map that the Lignitic group occupies a very large area 

 along the Upper Missouri and the Yellowstone Rivers, that it extends 

 far north into the British possessions. We may then trace it south- 

 ward in a broad continuous belt across the Yellowstone River, between 

 the Black Hills and the Big Horn Mountains, until it is overlapped by 

 the White River group, about sixty miles north of Fort Laramie. If 

 we continue southward along the east base of the Laramie range, we 

 find that the Lignitic group re-appears about ten miles south of the 

 Union Pacific Railroad. We find that where the White River group 

 and the Lignitic group come in contact, the former is superimposed on 

 the latter, and that really the White River group formed a vast basin 

 subsequent to the existence of the great lake in which the lignitic 

 sediments were deposited. We find also, by examining the White River 

 group along the base of the mountains, that the Laramie range formed 

 a barrier that prevented it from extending into the Laramie Plains ; 

 but the evidence is clear that, at the time of the existence of the 

 great Lignitic lake or sea, this barrier did not prevent the water- 

 communication with the Laramie Plains. Indeed, the evidence seems 

 quite clear that, with the exception perhaps of some isolated peaks ris- 

 ing above the waters, there was no mountain-barrier where we now 

 have the Laramie range. Therefore, with the exception of the Bear 

 River and Coalville group, we may connect the coal-bearing beds of the 



